1. Field of the Invention
This invention relates to local area network communication systems and medium adapters for use therewith, and more particularly to a ring/bus adapter for supporting multiple LAN topologies and protocols, including variable topologies, using installed ring media topology, and to resulting communication systems and methods.
2. Description of Related Art
Communication systems for interconnection of multiple users within a given geographic boundary, commonly referred to a local area networks (LANs), have found widespread acceptance in many fields for many uses. Initially developed for transmission of data between information processing equipment and related peripherals, LANs are now being used for distribution of voice communication, facsimile transmission, video communication, and machine and process control, among others.
No longer limited to laboratories or data processing centers, LANs are finding increasing use in business office communication, campus interconnection, workstation support, and industrial process and manufacturing control. For example, within an office environment LANs may be used to provide voice (e.g. telephonic) communication, as well as to link workstations and peripherals within the office. In an industrial setting, LANs may be used to communicate process or control commands between automated manufacturing equipment, design workstations, and controllers, as well as to provide other needed data and voice communication.
Typically these differing needs have been answered by providing separate and specialized communication systems for each application, utilizing differing LAN topologies and control protocols. Two basic media topologies are presently in widespread use, bus interconnection and ring interconnection. In a bus configuration, when one station transmits, all other stations receive the transmission. Buses may be considered as passive systems having a continuous effective transmission medium, in which transmissions are available at each connected station. In a ring configuration, in contrast, a station transmits only to the next connected station, which in turn transmits to a third station. A ring may be considered, therefore, as an active system utilizing unidirectional transmissions. Other topologies are also known, which are combinations of these two and possibly with interposed active circuits, but generally suffer from restrictions limiting their usefulness for the applications discussed.
In part because of the fundamental difference between these two basic media topologies, ring and bus, and in part because of differing user requirements for specific applications, several different sets of governing rules, or protocols, have been developed for controlling LAN communication. Principal among these are the three presently recognized standards of the Institute of Electrical and Electronics Engineers (IEEE), as follows:
IEEE 802.3: Bus Collision sensing multiple access with carrier detection (CSMA/CD), e.g. Ethernet;
IEEE 802.4: Bus Token passing, e.g. data link portion of
Manufacturing Automation Protocol (MAP); and
IEEE 802.5: Ring Token passing, e.g. International Business Machines (IBM) (trademark) Token-Ring Network.
Other access methods, such as method for providing isochronous communication over ring topologies, are also possible utilizing time division multiplex (TDM) and/or frequency division multiplex (FDM) schemes (see, e.g., Brandsma et al., "Philan: a Fiber-Optic Ring for Voice and Data," IEEE Communications Magazine, Vol. 24, No. 12, December 1986).
While these standards and associated communications systems have provided the user with solutions to many specific needs, systems themselves are largely incompatible. Thus a computer interface board designed to support the IEEE 802.3 standards cannot be used with an existing LAN including an incompatible transmission medium or media topology, such as an existing IEEE 802.5 token-ring system. This greatly complicates attempts to provide a "universal" LAN within given area, since protocols and related hardware particularly suited to one function may not be best suited to other uses. It is an object of the present invention, therefore, to provide a medium adapter which is capable of being adapted for use with numerous protocols, including the existing IEEE standards, and with existing interface hardware for implementing them.
Present incompatibilities cause serious problems when selecting LAN wiring to support multiple systems or topologies increases costs several fold. Thus a single LAN medium in a chosen topology is typically selected to meet present needs. Yet needs in a given area may change over time, and the needs in different areas, such as those of different tenants in a building, may differ widely. Since an installation is costly and preferably permanent, election of any existing system seriously impairs both present and future flexibility. For example, a building wired for IEEE 802.5 token-ring LAN cannot now be easily adapted to support bus LAN protocols or 802.3 interface-equipped stations. Similarly, existing hardware for use with one protocol, such as 802.3 interface boards, cannot now be used with other wiring installations, such as 802.5 token-ring wiring. This limits ability to upgrade wiring, or to use existing equipment which represent large investment, with alternative media. It is an important object, therefore, to provide a LAN medium adapter that can be used with a number of existing LAN media and wiring schemes, yet is capable of supporting both ring and bus communications under a number of differing protocols. A related object is to provide a highly flexible LAN communication system and method having these and other features.
Even within a given area, specific needs often change for different times of day or periods of days. For example, it may be desirable for an office LAN to efficiently support interactive data terminals or voice communication during the day, but efficiently transmit large quantities of data between only a few data processing stations at night. Unfortunately, existing network topologies and/or protocols which are best suited for one use may not be ideally suited to the another, requiring compromise in overall efficiency or duplicate systems. It is an object of the present invention to provide a communication system which may be selectively adapted without physical modification to support multiple topologies and protocols, making advantages of each available at different times on the same system.
Further, all existing LAN systems have limitations which users have heretofore accepted. For example, while for certain applications bus topologies offer advantages over rings, there has existed no means for determining whether or not a signal transmitted over the bus is actually propagated and presented to each receiver without distortion. Although protocols exist for collision detection, they may not be responsive, for example, to "lost" signals which fade below reception thresholds or to data corruption that does not effect the legality of the signal. In ring systems, failure of either the receiver or transmitter at a single station may destroy the entire LAN unless costly bypass circuits are provided. Important objects of the present invention are to overcome these and other shortcomings and provide a new LAN topology and system, including a medium adapter, with unique advantages.
Finally, the cost of providing each station of a LAN with necessary interface and adapter circuits has been a limiting factor on widespread adoption of LANs. Thus, a further important object is to provide the benefits discussed above by an inexpensive medium adapter without costly or complex hardware. A related object is to provide such an adapter which is highly reliable, and which may easily be incorporated into an existing LAN installation.